carving from side to side, wandering back and forth across the valleys. the valleys then cease to be v-shaped, become somewhat more flat-floored, and eventually all of the original upland surface is consumed by erosion from tributaries, so that all of the landscape is now in hill slope. this is what davis considered to be the stage of maturity. and finally, as the stream works down, it reaches near base level, or the limit to which it can erode downward and then does very little vertical erosion, expending almost all of its energy eroding from side to side. and as it does so, these valley bottoms become more and more broad, the tributary streams erode their slopes, eventually creating flood plains of their own, and, finally, virtually all of the material that was uplifted is destroyed and brought to an equal level, which davis referred to as a peneplain, as the ultimate stage in the cycle of erosion which he thought ended in "old age."